The geological time scale divides the last ~530 million years into 12 periods.
Fossils differ in the degree to which smaller structural details are preserved
and in the degree of chemical alteration of the original material. The
fossil record allows us to test various evolutionary theories against
the actual long-range history of life on Earth.
Geologic time scale: The Earth is about 4.6 billion years old, but
only the last (approximately) 530 million years is well documented by fossils.
Precambrian Era (up to about 530 million years ago,
sometimes divided into Azoic and Archaeozoic):
Includes the earliest fossils, about 4.2 million years old.
Precambrian fossils are very rare; most are microscopic fossils
of procaryotic organisms.
Palaeozoic Era (up to about 250 million years ago): The
time when invertebrates dominated the oceans and when fishes,
insects, amphibians, and land plants first flourished. Divided
into 7 periods:
- Cambrian (oldest)
- Ordovician
- Silurian
- Devonian
- Mississippian (=lower Carboniferous)
- Pennsylvanian (=upper Carboniferous)
- Permian (most recent)
Mesozoic Era (up to about 65 million years ago): Sometimes
called the "Age of Reptiles" because dinosaurs and other large reptiles
dominated the land while marine reptiles (and ammonoid mollusks) flourished
in the seas. Mesozoic time is divided into 3 periods:
- Triassic (oldest)
- Jurassic
- Cretaceous (most recent)
Cenozoic Era ("Age of Mammals"): The last 65 million years, divided
into 2 periods:
- Tertiary (from about 65 to 2 million years ago)
- Quaternary (the last 2 million years, including Pleistocene
and Recent epochs)
Paleontology: The study of fossils.
Fossils: Remains or other evidence of life of past geologic ages.
- Fossils containing original material:
- Unaltered remains. Example: frozen mammoths
- Compressions: Flattened and dehydrated, but unaltered
otherwise, with cellular details often preserved.
- Replacement fossils (with original material largely replaced):
- Permineralization, impregnation, and embedding:
Gradual addition of minerals by ground water, preserving
many internal details.
- Carbonization: Volatile compounds lost, leaving carbonized
skeleton only.
- Mineralization: Complete replacement of original material
by minerals.
- Casts and molds: Impressions in fine-grained sediments,
preserving only surface shapes. Casts are solid objects;
molds are hollow.
- Trace fossils: Tracks, trails, footprints, burrows, and
other traces of activity. Examples: Amber (fossil
tree sap or resin); coprolites (fossil dung).
Lessons learned from studying the fossil record: The fossil record can be
used to test various theories against the actual record of life on Earth.
No proposed theory or evolutionary mechanism is acceptable if it conflicts
with this historical record.
- The fossil record shows a historical process of branching and
diversification, not just a single linear sequence. "Evolution is
a bush, not a ladder."
- Cope's rule: Size increases frequently and decreases far less often.
- Williston's rule: Repeated parts (like multiple legs or segments)
often become less numerous and more different from one another.
- Dollo's law: Like other historical processes, evolution never
repeats itself exactly. Because of probability considerations, small,
simple changes may reverse, but larger and more complex changes never do.
Evolution is thus constrained (limited) by its own history.
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